Fix a issue that can be reproduced with:
1. Connect KGTP from remote and do a normal trace.
2. Do a normal off-line debug from remote and translate gtpframe file back to debug host.
3. Try to connect KGTP from remote host.  You will get fail.
This issue is because when function gtp_open alloc gtp_rw_buf, it doesn't reset gtp_rw_bufp and gtp_rw_size.
Add code reset gtp_rw_bufp and gtp_rw_size in function gtp_open fixed this issue.